1. Field of the Invention
The present invention relates to an alternator in which alternating voltage is generated in a stator by rotation of a rotor.
2. Description of the Related Art
In recent years, there has been an increasing demand for an automotive alternator that generates more power because of increasing vehicular load, whereas a mounting space therefore is decreasing because of a trend toward a smaller vehicular engine room.
In addition, there has been another increasing demand for reduced noises both inside and outside a vehicle, and engine noises are being decreased to respond to such a demand. A noise of an automotive alternator, which constantly operates to generate power to supply electrical load to a vehicle, has been a problem to be solved in achieving reduced noises. In the automotive alternator rotationally driven over a relatively wide revolution region from low speed to high speed, a wind noise or an electromagnetic noise thereof has been a topic to be tackled. Especially high-frequency wind noises or electromagnetic noises in a low engine speed region to a normal working region have been posing a problem because they sound particularly uncomfortable to the ears, having different frequencies from those of noises of other engine or engine accessories.
FIG. 14 is a sectional view of a conventional automotive alternator (hereinafter referred to simply as xe2x80x9calternatorxe2x80x9d), FIG. 15 is a perspective view of a rotor of FIG. 14, FIG. 16 shows a stator of FIG. 14 (a lead wire and a neutral wire of a stator winding are not shown), and FIG. 17 is an electrical circuit diagram of the alternator of FIG. 14.
The alternator includes: a case 3 composed of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 rotatably disposed in the case 3 and which has a pulley 4 secured to one end thereof; a Lundell-type rotor 7 secured to the shaft 6; fans 5 secured to both axial ends of the rotor 7; a stator 8 secured to an inner wall of the case 3; a slip ring 9 secured to the other end of the shaft 6 and which supplies electric current to the rotor 7; a brush 10 that slides in contact with the slip ring 9; a brush holder 11 accommodating the brush 10; a first rectifier 12a and a second rectifier 12b electrically connected to the stator 8 to convert alternating current generated in the stator 8 into direct current; a heat sink 17 fitted on the brush holder; and a regulator 18 adhesively fastened to the heat sink 17 and which adjusts a magnitude of an alternating voltage generated in the stator 8.
The rotor 7 is equipped with a field winding 13 for generating magnetic flux by passing an electric current, and a pole core 51 covering the field winding 13 in which magnetic poles are produced by the magnetic flux. The pole core 51 has a pair of a first pole core assembly 20 and a second pole core assembly 21 that intermesh with each other. The first pole core assembly 20 and the second pole core assembly 21 are made of iron and have claw-shaped magnetic poles 22 and 23 at their ends. Spaces 50 are formed between adjacent claw-shaped magnetic poles 22 and 23 to prevent magnetic flux from leaking from between the claw-shaped magnetic poles 22 and 23, and also to function as cooling passages for cooling the field winding 13.
The stator 8 is provided with a stator core 15 and a stator winding 16. The stator winding 16 has two windings, namely, a first three-phase stator winding 52 and a second three-phase stator winding 53, in which conductors are wound onto the stator core 15 with a phase difference of a 30-degree electrical angle.
The stator core 15 shown in FIG. 18 is formed by punching steel sheet into a comb-like plate with equidistantly arranged teeth, and by winding or laminating the comb-like plate into an annular shape. The stator core 15 has an annular core back 55, and a plurality of teeth 54 in which a plurality of slots 15a and openings 15b are formed, the slots 15a and the openings 15b radially extending inward from the core back 55 and being disposed equidistantly in a circumferential direction.
In this example, the stator core 15 includes the two windings, namely, the first three-phase stator winding 52 and the second three-phase stator winding 53, and the rotor 7 has sixteen magnetic poles with two three-phase portions corresponding to each pole. There are 96 slots 15a, openings 15b, and teeth 54, which are formed at regular pitches of 3.75-degree mechanical angles.
The first three-phase stator winding 52 and the second three-phase stator winding 53 have a front coil end 16a and a rear coil end 16b respectively projecting from both end surfaces of the stator core 15. The coil ends 16a and 16b are composed of a plurality of extending portions 30a, which are heat radiating portions. The extending portions 30a having the same shape are arranged orderly in a circumferential direction in two rows apart from each other in a radial direction.
FIG. 19 shows a winding structure of a stator winding 56 for one phase of the three-phase stator windings 52 and 53. In the drawing, dark dots in circles in the slots 15a of the stator 15 denote conductors 30 that extend from the front bracket 1 to the rear bracket 2, and cross marks (x) in the circles in the slots 15a of the stator 15 denote the conductors 30 that extend from the rear bracket 2 to the front bracket 1.
The stator winding 56 for the one phase is formed of the copper conductors 30, each of which has its outer surface coated with enamel. The conductors 30 of the first three-phase stator winding 52 are wave-wound at every six slots from slot No. 1 to slot No. 91. In each slot 15a, the conductors 30 are radially arranged in four layers in one row. In the stator windings for the remaining two phases of the first three-phase stator winding 52, the conductors 30 are wave-wound at every six slots from slot Nos. 3 to 93, and slot Nos. 5 to 95, respectively, and the conductors 30 are radially arranged in four layers in one row in each slot 15a. 
The conductors 30 of the second three-phase stator winding 53 are wave-wound at every six slots from slot No. 2 to slot No. 92. In each slot 15a, the conductors 30 are radially arranged in four layers in one row. In the stator windings for the remaining two phases of the second three-phase stator winding 53, the conductors 30 are wave-wound at every six slots from slot Nos. 4 to 94, and slot Nos. 6 to 96, respectively, and the conductors 30 are radially arranged in four layers in one row in each slot 15a. 
Furthermore, as shown in FIG. 17, the stator windings 56 for the three phases are star-connected to form the first three-phase stator winding 52, and the additional stator windings 56 for the three phases are also star-connected to form the second three-phase stator winding 53. The three-phase stator windings 52 and 53 are provided in the slots 15a with a phase difference of a 30-degree electrical angle from each other, and are electrically connected to the first rectifier 12a and the second rectifier 12b, respectively. Direct current outputs of the rectifiers 12a and 12b are connected in parallel and combined.
In an automotive alternator of the above construction, current is supplied by a battery (not shown) through the brush 10 and slip ring 9 to the field winding 13 so as to generate magnetic flux, whereby the claw-shaped magnetic poles 22 of the first pole core assembly 20 are polarized with north-seeking (N) poles, while the claw-shaped magnetic poles 23 of the second pole core assembly 21 are polarized with south-seeking (S) poles.
The pulley 4 is rotated by an engine, and the rotor 7 rotates together with the shaft 6. This causes a rotating magnetic field to be imparted to the three-phase stator windings 52 and 53, and an electromotive force is generated. The alternating electromotive force is converted into direct current by means of the rectifiers 12a and 12b, a magnitude thereof is adjusted by the regulator 18, and the battery is recharged.
In the automotive alternator having the construction described above, there are two slots per pole per phase, the first three-phase stator winding 52 and the second three-phase stator winding 53 are incorporated in the stator core 15 with the 30-degree electrical phase difference, and measures against output surges are provided. Similar technical contents are disclosed in Japanese Unexamined Patent Publication No. HEI4-26345.
However, in the automotive alternator having the above construction, a magnetic attractive force is produced between the rotor 7 and the stator 8 during power generation because of an interaction between a rotating magnetic field including higher harmonics produced by the claw-shaped magnetic poles 22 and 23 of the rotor 7 and an alternating magnetic field including higher harmonics generated from alternating current produced at the stator winding 16, and an interaction between a rotating magnetic field including higher harmonics generated from the claw-shaped magnetic poles 22 and 23 of the rotor 7, and permeance higher harmonics generated by the stator slots 15a. The electromagnetic attractive force leads to an electromagnetic exciting force of the claw-shaped magnetic poles 22 and 23 of the rotor 7 and the stator core 15, with consequent occurrence of vibrations and electromagnetic noises.
Reducing the vibrations and noises requires the electromagnetic exciting force be reduced. For this purpose, it is important to reduce a magnetomotive force higher harmonics and slot higher harmonics produced in the stator 8. It is particularly important to reduce fifth, seventh, eleventh, and thirteenth higher harmonics, which are large among the magnetomotive force higher harmonics of the stator 8, and the eleventh and thirteenth higher harmonics, which are large in the slot higher harmonics.
In the automotive alternator of the above construction, as will be discussed hereinafter, when a basic frequency of output current is denoted as f, it is possible to reduce a sixth higher harmonic component of a 6f frequency. However, a twelfth component is large, and there are two slots 15a per pole in a phase; therefore, the twelfth component of a rotor magnetomotive force higher harmonic agrees with the number of the slots 15a, so that electromagnetic noises due to stator slot higher harmonics accordingly increase. Furthermore, air interference noises corresponding to a 12f frequency caused by the slots 15a are increased, and the wind noises and the electromagnetic noises interfere with each other, producing uncomfortable noises. The 12f-component noise, in particular, corresponds to a 96th component if the rotor 7 has sixteen poles when it is converted into a rotational order ratio of a number of revolutions. Thus, in a range of number of revolutions from 2000 to 6000 rpm of the automotive alternator, which corresponds to an engine speed in a range from idling speed to a normal speed, noises reach a range of frequencies from 3.2 kHz to 9.6 kHz when the rotor 7 has, for example, sixteen poles. This frequency range covers a high-frequency band of 2 to 10 kHz, to which the ears are most sensitive, posing a problem in that the noises are extremely unpleasant to passengers.
The present invention aims to solve the problem set forth above, and an object of the present invention is to provide an alternator capable of reducing unpleasant high-frequency noises.
To this end, according to one aspect of the present invention, there is provided an alternator in which irregular intervals are provided between centerlines extending in a radial direction of openings of adjoining slots, and a first three-phase stator winding and a second three-phase stator winding are wound around a stator core with a phase difference of an electrical angle of 31 to 34 degrees.
In a preferred form of the alternator in accordance with the present invention, a circumferential width of a tooth is set such that both ends of a tooth located between adjoining claw-shaped magnetic poles overlap proximal ends of the two claw-shaped magnetic poles, as observed from a radial direction.
In another preferred form of the alternator in accordance with the present invention, side surfaces of the claw-shaped magnetic poles have chamfered portions.
In yet another preferred form of the alternator in accordance with the present invention, the three-phase stator windings have star connections, and neutral points of the star connections are electrically connected to rectifiers for rectifying ac output.
In a further preferred form of the alternator in accordance with the present invention, circumferential widths of the openings of the slots are regular, while circumferential widths of distal ends of adjoining teeth are irregular.
In a further preferred form of the alternator in accordance with the present invention, circumferential widths of the openings of adjoining slots are irregular, while circumferential widths of distal ends of the teeth are regular.
In a further preferred form of the alternator in accordance with the present invention, the first three-phase stator winding and the second three-phase stator winding are wound around the stator core with a phase difference of an electrical angle of 32.5 degrees.